Stephanie is a single mother with a parenting vlog who befriends Emily, a secretive upper-class woman who has a child at the same elementary school. When Emily goes missing, Stephanie takes it upon herself to investigate.

Circa 1969, several strangers, most with a secret to bury, meet by chance at Lake Tahoe's El Royale, a rundown hotel with a dark past. Over the course of one night, everyone will show their true colors - before everything goes to hell.

An environmental analyst is asked by the police to determine if two deaths by hydrogen sulfide poisoning are an accident - or a murder. But when he meets a young woman at both sites, a scientific mystery begins.